Because none of my sports shorts fit (a.k.a. My single pair of sports shorts I bought pre-kids is about 3 sizes too small). My sports bra is about 2 cup sizes too small and the t-shirt is suffering the same fate.
I have some hiking shorts that more or less fit and I have an old school ( 
that I left 20 + years ago) aertex shirt. Or some equally ancient touristy t-shirts. I usually wear these but as I never exercise outside, it's never mattered what I wear.
Will I be laughed at tomorrow? I can't believe it didn't occur to me before